Why Did The Tornado Outbreak Happen?

By Terry Eliasen, WBZ-TV Executive Weather Producer

BOSTON (CBS) – June 1, 2011 will be a day that we will forever remember in New England. It is now etched in our minds along with other infamous weather dates like “The April Fool’s Day Blizzard” in 1997, “The Perfect Storm” in 1991, Hurricane Gloria in 1985, and Hurricane Bob in 1991. Looking further back, we had the “Blizzard of 1978” and the Worcester Tornado on June 9, 1953.

Wednesday’s tornado outbreak, though, is likely the most violent weather event to occur in Massachusetts in many of our viewers’ lifetimes. This is truly a once in a lifetime event, and lets hope that it is the last time in this life!

Why Did The Tornadoes Happen?

Many folks are wondering why this happened. What was different in the atmosphere yesterday compared with severe weather events in the past? The answer is not clear-cut.

Tornado outbreaks like Wednesday truly are not forecast-able. We can predict a likelihood of tornadoes based upon a diligent forecast, and there was a tornado watch issued just before 1 p.m. Wednesday, but many times there are no tornadoes after a watch is issued. Several factors have to line up perfectly for a severe weather outbreak like the one we experienced yesterday to occur. These are factors that are very rarely seen here in New England, more often in the Midwest.

WBZ-TV’s Barry Burbank explains the tornadic outbreak and the E-F scale:

On a basic level, here is how things unfolded in the atmosphere:

A warm front came through the area during the early morning hours. Many of you may remember waking up or driving through some nasty downpours and hail around 9 a.m. These thunderstorms quickly raced offshore and very warm and humid air rushed in on southwest winds. The sun re-emerged and caused temperatures to rise sharply into the 80s, thus providing the spark the atmosphere needed.

At the same time, a cold front was rushing from northwest to southeast from New York State towards New England. This front represented a change in airmasses from the warm and humid over our area to a much drier and cooler Canadian airmass. This cold front provided the lift needed to create large thunderstorms.

A cold front is a wedge of cold, dense air…it literally cuts underneath the warm and moist air, forcing it to rise quickly up into the atmosphere. When warm air rises into cooler air, it condenses and forms clouds. When you have large amounts of very moist air rising very sharply, you get very large clouds (cumulonimbus) which become thunderstorms.

Wednesday’s atmospheric conditions were ideal for all of these events to occur, but you still need more than that for tornadic supercells to form.

Three Key Factors

Gonna get a bit technical here, but there were at least three other factors in play that were crucial in turning these thunderstorms into deadly tornadoes.

1) The Lifted Index: This is a model forecasted number we look at as meteorologists during severe weather. Lifted Index is one of the most common ways to measure how unstable the atmosphere is. The value is obtained by computing the temperature that air near the ground would have if it were lifted to a higher level (around 18,000ft typically), and comparing THAT temperature to the actual temperature at that level. Negative values indicate instability…the more negative the number the more unstable the air. A typical severe weather day we may see LI’s somewhere between -1 and -5, but on Wednesday, the LI was in the range of -8 to -10.

2) CAPE: This is another important meteorological term, which is short for Convective Available Potential Energy. Simply put, this is the measure of the amount of energy available in the atmosphere for convection (severe weather). CAPE is directly related to the maximum potential vertical speed within an updraft (in other words, how fast can the air rise within a thunderstorm); thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1,000 joules per kilogram (j/kg), and in extreme cases may exceed 5,000 j/kg. Wednesday’s values approached 4,000 joules in Western Massachusetts.

3) Helicity: This is one more very important meteorological factor, especially with tornadic development. This one is perhaps the most difficult to explain. It is a measure of spin in the atmosphere and is measured by looking at a vertical wind profile of the atmosphere. The more wind shear you have (wind blowing at different speeds and directions in different layers of the atmosphere), the greater the rotation within a storm. Generally you are looking for a helicity number of 150 or greater for any type of significant rotation, and Wednesday’s readings were between 200-300 in the bottom 3-km of the atmosphere.

So clearly, Wednesday was a day where just about everything lined up perfectly in the atmosphere, and we see the amazing and devastating results that can occur when this happens. It should serve as a reminder that while these events are rare, we should never take the threat of severe weather lightly and should always take the utmost precaution to protect our lives and the lives of those around us. Even the smallest of thunderstorms can be deadly.

More from Barry Burbank
Comments

One Comment

  1. JimmyJames says:

    Great blog and thanks for the explanation of what happened yesterday. The pictures are something you would see out of the Midwest. Hopefully we will not have anymore episodes of severe weather this summer like we saw yesterday.

  2. David White says:

    Thanks for the great blog. Could there be a connection between the unusually high shear and the following:

    La Nina decaying towards La Nada
    Positive NAO soon to trend neutral if not negative
    The lowest sunspot activity current, since the 1930s
    The recent PDO shift from warm to cold
    Or closer to home – Atlantic Multidecadel osciallation – warm at present
    MJO activity – relation between that and hurricane activity

    I would like to see this summer many days like today and what is forecasted for the weekend. Perhaps we can get three or four weeks of those having come out of three or four weeks of first cool and damp, then warm and humid, and the severe thunderstorms/tornados. Mother nature always balances!

    1. Joe says:

      Barry, as usual a thorough and informative blog! You always have a way of respectfully “dumbing down” the intricate weather sciences involved within any weather event for the average reader to understand. I find it amazing that all the necessary tornadic conditions you mentioned came together at just the right time. I have a few questions. I know your very busy so I understand if you don’t have time to answer them.

      * “The value is obtained by computing the temperature that air near the ground would have if it were lifted to a higher level (around 18,000ft typically), and comparing THAT temperature to the actual temperature at that level.”

      When you say “at that level” are you referring to the ground level temperate? Meaning, a lifted index of -10 would occur if it was 80 at ground level and 70 at the higher level? Or is there something else involved than just the difference between the ground temp and upper level temp (ie. LI= Upper Level Temp – Ground Temp)?

      Also, how is CAPE calculated? If I remember correctly a joule is related to the amount of energy or force needed to move an object a certain distance. To be honest, Im guessing but I think I’m in the ballpark. Are the joules calculated by the speed, in mph, of the lift or is it related to the density of the air mass being lifted?

      Finally, am I right to assume that the lifted index and CAPE are calculated first and those values are then used in determining the helicity value? admittedly I have no clue of what I’m talking about but I have always found the science behind weather and forecasting to be very interesting. The fact that the science allowed for a tornado warning of even a few minutes saved many many lives. You guys take a lot of heat for being even slightly off on a forecast. When it mattered most, yourself and every met involved not only got it right but ultimately saved lives and deserve a big thank you from a lot of people.

      Thank you,

      Joe

      1. Charmed says:

        It looks like Terry Eliasen wrote this, not Barry. I may be wrong but “Why Did The Tornado Outbreak Happen?

        By Terry Eliasen, WBZ-TV Executive Weather Producer” is on the top of the page. But I wholeheartedly agree with you Joe then you said, “The fact that the science allowed for a tornado warning of even a few minutes saved many many lives. You guys take a lot of heat for being even slightly off on a forecast. When it mattered most, yourself and every met involved not only got it right but ultimately saved lives and deserve a big thank you from a lot of people. ” Reading all of the facebook comments and notes here I feel the same way! Some people are so harsh and I think it’s so uncalled for and down right nasty. There are so many monday morning quaterbacks out there its ridiculous. It looks like Terry Eliasen wrote this, not Barry. I may be wrong but “Why Did The Tornado Outbreak Happen?

        By Terry Eliasen, WBZ-TV Executive Weather Producer” is on the top of the page. But I wholeheartedly agree with you Joe when you said, “The fact that the science allowed for a tornado warning of even a few minutes saved many many lives. You guys take a lot of heat for being even slightly off on a forecast. When it mattered most, yourself and every met involved not only got it right but ultimately saved lives and deserve a big thank you from a lot of people. ” That was perfectly stated!

        Reading all of the negative Facebook comments and weather blogs I feel the mets took a lot of heat for Wednesday’s events! Some people are so harsh and I think it’s uncalled for and down right nasty. There are so many Monday Morning quarterbacks out there making so many unnecessary and ignorant comments that it makes me wonder what’s wrong with people today. Why do they feel the need to publicly bash people that are trying to help them by keeping them informed on a VERY dangerous situation? They are doing their jobs!

        By the way, I am not ignorant to the following facts; these folks choose to be meteorologists (on TV) and some of the ‘heat’ comes with the job and; when you are on a site like Facebook or Twitter you have to take the bad with the good. I just really think some people take their opinions (and their ignorance) too far, but then again, that’s just me and MY opinion.

        It seems to me that the WBZ weather team is highly educated, they work hard, they LOVE what they do and they work great as a team! It’s great to see when all of that comes out live, on the air like it did on Wednesday! They really did save lives! They were spot-on with forecasting, they kept up with the Tornado watches and warnings and were repeatedly telling people what to do just in case a tornado came through their area. I thought their coverage was amazing; I was glued to WBZ on the TV and on the Internet all afternoon. Kudos to the entire WBZ weather team and a big THANK YOU for all that you do everyday!

  3. Born in Oklahoma says:

    The only thing I would take exception to is the quote that “these type of outbreaks are truly un-forcastable”. They actually are if you know what you’re doing. Trust me, mets in Oklahoma can tell you in the morning when there’s clear blue sky, that there will be outbreaks of F3’s and F4’s that afternoon—and it actualy happens. So yes, these things can be forcasted. Those guys have equipment/radar that can tell you down to the block and street intersection where a tornado is. They also have an army of spotters out.
    Of course, those mets couldnt forcast snow amounts north and south of the pike, or where a rain/snow line will set up to save their lives. It all what you’re used to forcasting. But to say tornado outbreaks like that are unforcastable is’t true. You just have to know what to look at and have done it for 30 years.

  4. montreal web design says:

    Im studying to be a web designer so Im going to community faculty for 2 years and im going to transferre to NC state in 2014. i heard that web desgners make good money..

  5. helicopter licence says:

    We’re a gaggle of volunteers and starting a new scheme in our community. Your web site provided us with valuable information to work on. You have performed a formidable job and our entire community might be grateful to you.

Leave a Reply

Fill in your details below or click an icon to log in:

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

More From CBS Boston

WheelMobile
Download Our App

Listen Live